The invention relates to a magnetic-tape apparatus having an arcuate guide surface over which a magnetic tape is guided along a helical path over a specific circumferential angle .phi..sub.0 in order to read signals from inclined parallel tracks on the tape by means of a rotating magnetic head. Transport means move the magnetic tape over the guide surface with a nominal speed V.sub.0 and with variable speeds n.multidot.V.sub.0, the magnetic head describing a rotary path parallel to a reference plane during operation at the nominal speed V.sub.0. Detection means derive a control signal whose magnitude depends on the speed n.multidot.V.sub.0 of the magnetic tape relative to the guide surface. Adjustment means adjust the inclination of the rotary path of the magnetic head relative to the reference plane depending on the magnitude of the control signal, in order to scan the tracks by means of the magnetic head.
Such a magnetic-tape apparatus is known from U.S. Pat. No. 3,375,331. In this apparatus the head support is mounted on a pivotable drive shaft, the adjustment means adjusting the tilting angle of the drive shaft and hence the inclination .alpha. of the rotary path of the magnetic head relative to the reference plane depending on the magnitude of the control signal from the detection means, which depends on the speed of the tape over the guide surface. This tilting ensures that the head follows the track in an optimum manner in the case of tape speeds which deviate from the nominal speed at which the head has written the track on the tape. Without this tilting there will be a different inclination between the path of the head over the tape and the longitudinal axis of the track in the case of a deviating tape speed, as a result of which only a limited portion of the track can be read. Thus, tilting enables, for example, a passage on the tape to the located rapidly by tape transport at a comparatively high speed. As as result of the tilt of the drive shaft the head follows a sinusoidal path on the tape, i.e. the head performs a sinusoidal excursion in height relative to the reference plane. In principle a linear excursion in height is required to follow the inclined track and the sinusoidal path therefore gives rise to tracking errors during the head excursion. In the known apparatus, which dates back to a period in which comparatively large track widths of approximately 200 .mu.m were used, the sinusoidal approach for the track is acceptable because the maximum tracking errors for the sinusoidal path are comparatively small relative to such track widths, so that the head can follow the track over the entire length at a specific higher tape speed.
However, in recent years magnetic heads and magnetic tapes have been improved considerably and the use of digital-signal recording technology now enables tracks having a width of less than 10 .mu.m to be written and/or read. In the case of such comparatively narrow tracks tilting of the drive shaft is likely to give rise to such tracking errors that the head does not follow the track over the entire length, so that only a limited portion of the track can be read. In digital-signal recording systems such as the recently developed R-DAT (Rotrary Digital Audio Tape recorder), but also in the digital video recording systems now being developed, this may result in a correct reading of the digital data being impaired.
U.S. Pat. No. 3,375,331 discloses apparatus constructed as a video recorder. The tape is wrapped around the drum-shapedguide surface over a comparatively large circumferential angle greater than 180.degree., the track length being equal to half the drum circumference. The analog-signal recording technology used implies that each track should contain one picture field to enable the pictures to be reproduced. It is almost impossible to utilize a circumferential angle smaller than 180.degree., because with such a circumferential angle the track length is inadequate for one picture field. Thus, the known apparatus does not allow the maximum tracking errors occurring as a result of tilting of the drive shaft to be reduced by a reduction of the circumferential angle.